ESCA is a technique whereby a solid sample is irradiated with low energy photons causing photoelectrons to be emitted from surface atoms. The emitted photoelectrons are energy analyzed and detected, with the resulting signal acting as a "fingerprint" for elements on the sample surface. The instrument of the present invention measures the kinetic energies of the emitted electrons and from these measurements the elemental and chemical composition of the outermost about 20 atomic layers can be determined.
ISS is a technique whereby a solid sample is bombarded with monoenergetic ions and the energies of the scattered ions are determined. The instrument of the present invention measures the kinetic energy distribution of the scattered ions, and from this measurement, the elemental composition of the single outermost atomic layer can be determined.
It is known to use a movable aperture to restrict the area of analysis down to 250 micrometers. The position of this aperture is not at an image plane in the lens optics, but rather in front of the entire lens assembly. This orientation allows the area of analysis to change with kinetic energy of the detected photoelectrons or ions. However, this lack of constant area leads to data inaccuracies and difficulty in data interpretation. Chemistry from different sections of the sample can contribute to the detected signals, depending on the kinetic energy of the photoelectrons or ions associated with each chemical state. Furthermore, the known lens design does not allow the controlling computer to change the image magnification of the detected ions or photoelectrons. Thus, with the known design, the sensitivity of analysis, directly related to speed of analysis, decreases proportionally with the reduction in analysis area as the operator changes from the large area mode to the small area mode.
It is also known to use a focused x-ray source to define the area of analysis. Because only quartz (100) monochromators are viable for low energy x-ray applications, the monochromator approach forces the user to employ only A1 K.alpha.x-rays. This limits the chemical systems that can be studied in the small spot mode. In addition, due to the large diameter Rowland circles used in the monochromators of the known system such as, for example, 200-500 mm, the x-ray intensity at the sample surface is low. Although the x-ray flux is refocused through the x-ray monochromator, the resulting ESCA sensitivity is low for the focused x-ray source approach.